This invention relates generally to a method of and apparatus for electrically stimulating a heartbeat. More specifically, this invention relates to a method of implanting a permanent pacemaker bipolar lead apparatus, and an implantable permanent pacemaker bipolar lead apparatus.
Heartbeat pacing devices used to electrically stimulate a heartbeat in order to restore or control the rate and rhythm thereof are known as pacemakers, which generate electrical pulses of appropriate strength and duration for heartbeat stimulation. Such pacemaker electrical pulses are then conducted to a point or region of the heart muscle by means of conductors and electrodes comprised of suitable material and configured to make electrical contact with and couple to the heart muscle; such a combination of conductor and electrode is known as a lead system.
One form of pacemaker and lead system combination is known as a permanent pacemaker, used for long-term therapy. In employing permanent pacemakers, the pacemaker and lead system are implanted in the patient's body.
The most common form of implantable lead for a permanent pacemaker is known as a cardiac catheter, which comprises a flexible tube containing a conductive cable and an electrode at or near its distal end. In implanting a catheter lead, the distal end of the cardiac catheter is extended into the body and positioned in one of the chambers of the heart, with the electrode positioned to bear against the inner surface of the heart muscle. A form of implantable catheter lead used previously comprised a bipolar configuration in which the pacemaker's negative and positive terminals were each connected to a separate insulated conductive cable in the catheter, and each cable terminated in an exposed electrode at or near the distal end of the catheter. In the preferred arrangement of such bipolar catheter lead, the negative electrode was at the very tip of the catheter making direct contact with the interior surface of the heart muscle, and the positive electrode was positioned approximately one or two centimeters back from the catheter tip; electric current flowed between these two electrodes through the heart muscle and/or the blood in the heart chamber in which the electrodes were positioned. Since the heart muscle was in front of the negative electrode at the tip of the catheter, whereas electric current was directed to flow back towards the positive electrode, a portion of the electric current flowing between the two electrodes did not pass through the heart muscle, but rather traveled through the blood alone and was therefor without stimulating effect. In view of such loss of stimulating current, a greater electrical output was required from the pacemaker in order to reach the minimum current for stimulating the heart muscle, and more power was required by the pacemaker than was necessary for stimulation alone. In order to overcome the undesirable overconsumption of power frequently occurring with bipolar configurations, a unipolar configuration of implantable catheter lead was developed in which only the negative electrode was in the catheter at the tip thereof, and a separate positive electrode was implanted a variable distance from the heart to provide for the passage of a greater percentage of the total stimulating current through the heart muscle by creating a flow pattern of electric stimulating current with more flow lines or streamlines directed through the heart muscle. The most common form of positive lead in such systems was a large metallic plate on one surface of the pacemaker. Such a unipolar configuration proved to be only a slight improvement over the bipolar configuration. The substantial distance from the heart to the pacemaker, particularly when the pacemaker was implanted above the level of the heart in the vicinity of the shoulder or neck, permitted a substantial flow of electric current away from the negative electrode thereby preventing it from reaching the heart muscle. Both the bipolar and unipolar configurations permit an electric current flow pattern which diverges from the negative electrode so that current density within the heart muscle is less than anticipated in view of the total current produced by the pacemaker; therefore, the stimulating capacity of an electrical pulse from the pacemaker, dependant upon the current density produced by the electrical pulse within the heart muscle, is substantially diminished in such devices.